Sigil Captain
The trigger reads like a payoff hunting for an enabler, and the enabler is the whole appeal: this rewards a board built on the most disposable bodies in the game. Any creature that enters at exactly 1/1 (a token off a Saproling-maker, a mana dork, a fresh one-drop) arrives instead as a 3/3, and because the upgrade comes in counters rather than a continuous anthem, it survives effects that strip static bonuses and stacks with later pumps. The narrow condition is what keeps the rate in line: it cares only about creatures that enter at precisely 1/1, so anything that walks in bigger gets nothing, and the engine pays out only when you flood the board with the smallest possible threats. It sits squarely in the Selesnya tokens lineage, the color pair that has always wanted its go-wide swarms to mean something individually and not just in aggregate. The trigger fires per qualifying creature, so a single token-doubler or a mass-token spell can convert a fragile board into a wall of 3/3s in one beat. Built for the deck that treats 1/1s as raw material, it is inert in any list that does not, and that honesty about its lane is part of why the design reads so cleanly.
